Low density composite rocket nozzle components and process for making the same from standard density phenolic matrix, fiber reinforced materials

ABSTRACT

A process for producing low-density composite components as disclosed herein involves forming a compacted and curable pre-preg into a selected level of compaction whereby voids are capable of forming in the compacted curable pre-preg. The compacted curable pre-preg is then cured at a pressure sufficiently low to permit evolving gases to form voids in the pre-preg as the pre-preg cures into the composite article. The pre-preg is only partially debulked in the process. Rocket nozzle components may be produced with reduced densities while still exhibiting satisfactory erosion and other characteristics desired for products subject to the harsh erosive environment of a rocket motor.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.09/090,256, filed Jun. 4, 1998, pending, which claims the benefit ofeach of U.S. Provisional Application No. 60/048,605 filed on Jun. 4,1997, U.S. Provisional Application No. 60/048,604 filed on Jun. 4, 1997,and U.S. Provisional Application No. 60/048,672 filed on Jun. 5, 1997,the complete disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to low-density composite articlesand, in particular, to low-density rocket nozzle components. The presentinvention further relates to a process for making the low-densitycomposite rocket nozzle components.

[0004] 2. State of the Art

[0005] Solid rocket motor nozzle components have been fabricated usingconventional composite starting materials referred to as pre-pregs.Pre-preg materials generally include fabric and/or fiber that has/havebeen pre-impregnated with resin, typically a phenolic resin. The fabricor fiber is referred to as the reinforcement of the composite while theresin is called the composite matrix or matrix formulation.

[0006] Depending upon the position and function of the component in thenozzle and the intended application of the nozzle, either astandard-density or low-density material (pre-preg) may be used.

[0007] Historically, pre-pregs for fabricating standard-densitycomposite rocket nozzle components include the reinforcement, matrixformulation, and appropriate fillers. In the case of standard-densitycarbon or graphite cloth reinforcement and phenolic resin, carbon havingsubstantially the same density as the carbon fiber is selected as thefiller. Carbon or graphite fibers can be rayon, polyacrylonitrile(“PAN”) or pitch-based materials. Glass and silica composite pre-pregsutilize silica fillers when fillers are used.

[0008] To achieve a low-density composite (LDC) pre-preg, which isadvantageous in reducing motor weight of a rocket motor formedtherefrom, hollow spheres, such as described in U.S. Pat. No. 4,268,320,U.S. Pat. No. 4,294,750, or U.S. Pat. No. 4,621,024, are introduced intothe pre-preg formulation as the filler. The effective densities of thesehollow spheres typically range from 0.2 g/ml to 0.5 g/ml. To prevent thehollow spheres from clumping in the pre-preg, an elastomer is added tothe resin mix to maintain a more even dispersion of the hollow spheresduring pre-impregnation of the fiber/fiber reinforcement. However, dueto the expense associated with hollow spheres and the elastomer added tothe resin mix, as well as other known difficulties in producing thelow-density pre-preg, the cost of the conventional low-density compositematerial can be 50 to 100 percent higher than that of thestandard-density version of the material.

[0009] The inclusion of hollow spheres and elastomer in the pre-pregformulation also results in a composite having an across-ply tensilestrength as low as one-tenth that of standard-density material. Thelower across-ply tensile strength of LDCs significantly increases thelikelihood of the LDC rocket nozzle components experiencing ply lifting,wedge outs and other failure phenomena. LDCs used in exit coneenvironments can exhibit ply lift. The tendency of these materials toexhibit these failure modes must be addressed and accommodated by nozzledesign. Such accommodation typically involves making the componentsthicker to improve margins of safety; however, the added thickness ofthe components partially offsets the weight advantage, i.e., the lowerdensity, that LDC materials have over standard-density materials.

[0010] One predominantly used process for the fabrication ofconventional so-called standard-density nozzle components involvesapplying material to a mandrel such as by tape wrapping; ply-by-plyapplying and debulking of pre-preg at very high pressures andtemperatures to soften the resin, immediately followed by cooling; andautoclaving or hydroclaving curing, such as by pressurized curing at 200to 1000 psig. The material is applied to the mandrel in such a way as toachieve 80 to 95 percent of the material debulk (compaction) required inthe final component. Currently practiced processes can require apressure greater than 800 psig to 2400 psig, to achieve desireddebulking. Final debulking is achieved during the pressurized cure. Thisprocess provides a cured composite specific gravity (SpG) incarbon/graphite phenolic components of 1.40 to 1.60, glass phenoliccomponents of 1.95 to 2.05 and silica phenolic components of 1.70 to1.80 (g/ml).

[0011] The conventional pre-pregs are designed to be used at elevated(high) pressures and temperatures to produce fully densified composites.

[0012] Each of the above-mentioned processes has different drawbacks,some of which are noted above. In particular, the art has soughtlow-density composite rocket nozzle components which are produced atlower average per unit cost, but which are capable of exhibiting theerosion resistance, charring resistance, and across-ply tensilestrengths of a comparable rocket nozzle component made by theconventional process with a standard-density composite.

BRIEF SUMMARY OF THE INVENTION

[0013] The present invention provides a process which overcomes theabove-mentioned and other drawbacks associated with the conventionalmanufacture of composite rocket nozzle components and addresses theneeds expressed above, while affording a reduction in manufacturingcosts.

[0014] The present invention also provides articles, such as compositerocket nozzle components, that synergistically combine the excellentphysical properties and low production costs of standard-density nozzlecomponents with the reduced weights of LDC nozzle components, even whenthe components are substantially or completely devoid of low-densityfillers (microballoons etc.).

[0015] The present process achieves the aforementioned advantages whileenabling the practitioner to avoid the need to use a specially designedpre-preg to fabricate low-density composite articles, including rocketnozzle components.

[0016] These and other features and advantages of the present inventionwill become apparent from the following detailed description when takenin conjunction with the accompanying drawings which illustrate, by wayof example, the principles of the present invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0017] The accompanying drawings illustrate the present invention. Insuch drawings:

[0018]FIG. 1 is a flow chart comparing a process for preparingconventional “standard-density” composite articles, the present processfor preparing low-density composite articles, and an industry standardprocess for preparing low-density composite articles;

[0019]FIG. 2 is a diagrammatical view of a tape wrapping processsuitable for practicing an embodiment of the present invention;

[0020]FIG. 3. is a schematic showing a press debulking step practiced inaccordance with another embodiment of the present invention;

[0021]FIG. 4A is a cross-sectional view of a forty pound charge motorhaving propellant loaded therein;

[0022]FIG. 4B is a sectional sliced side view of a forty pound chargerocket motor (“FPC”) which is useful for testing nozzle materials;

[0023] FIG 4C is a graph showing the relationship of time and pressure;

[0024]FIG. 5 is a side view of a platen press showing the vacuum bag andbleeder material;

[0025]FIGS. 6A and 6B comprise a table summarizing results obtained withcomposites;

[0026]FIG. 7 is a bar graph reporting total heat affected depths offorty pound charge (FPC) motor 45° blast tube sections;

[0027]FIG. 8 is a bar graph reporting total heat affected depths offorty pound charge (FPC) motor 30° blast tube sections;

[0028]FIG. 9 is a bar graph reporting some normalized erosion rates ofaft exit cone section test specimens;

[0029]FIGS. 10A and 10B comprise a table reporting a comparison of tagend properties with FPC billet performance;

[0030]FIG. 11 is a table of results from testing of composite materials;

[0031]FIG. 12 is a graph showing a comparison of average across-plythermal expansion as a function of temperature (10° F./sec);

[0032]FIG. 13 is a bar graph of a comparison of ultimate torsional shearstrength of modified materials;

[0033]FIG. 14 is a bar graph of a comparison of fill permeability ofmodified composite materials;

[0034]FIG. 15 is a bar graph of a comparison of open porosity ofmodified composite materials;

[0035]FIG. 16 is a bar graph of fill thermal expansion maximum peakheight of modified materials; and

[0036]FIG. 17 is a cross-sectional view of a part of a rocket motornozzle component.

[0037] In the Figures, various designations may be used. UF meansunfilled. VC means vacuum cured according to the present invention.“NARC HRPF” means standard-density composite made using a pre-pregMX-4926 from Fiberite. An “LDR” designation, such as in FIG. 7, meansspecially formulated pre-preg designed for use in fabricating aconventional low-density composite having microballoon fillers. Thedesignation “PC” means a post-cure step.

DETAILED DESCRIPTION OF THE INVENTION

[0038] The present process concerns the production of compositereinforced products, particularly rocket nozzle type components. Thenovel rocket nozzle components have lower density compared tocorresponding industry standard-density composite reinforced rocketnozzle components. The rocket nozzle components, however, remarkablyexhibit erosion resistance and charring which are similar to thehigh-density products prepared using an industry standard practice.Thus, in addition to cost savings, the process offers a reduction in theweight penalty and the retention of desirable erosion and charringproperties in finished rocket nozzle components.

[0039] In general, the process comprises (a) forming a compactedpre-preg into a desired configuration under a predetermined level ofcompaction which is less than the compaction (pressure) applied in theconventional process; (b) curing the formed pre-preg at a selectedtemperature and pressure, wherein the pressure may be vacuum or lessthan about atmospheric pressure (e.g., about 14 lbs/sq. inch at sealevel); and (c) obtaining the cured product, such as a low-densityrocket nozzle component. In the process the pre-pregs are partiallydebulked and, in general, may be debulked less than 80% or, moreparticularly, debulked to less than about 50%. Therefore, although theterms “debulked” or “debulking” may be used in describing the invention,it should be understood that what is meant is “partially debulked,” e.g.less than 80%, and preferably less than 50%, debulked.

[0040] In accordance with the process, the forming and curing areconducted at sufficiently low pressures to permit the formation of voidsin the resulting component, the voids being formed from volatilesevolving during the curing. Hence, the resulting cured product has alower density than standard-density products, even when the curedproduct of the present invention is formed from a compositionsubstantially or completely free of low-density fillers, such as hollowspheres (“microballoons”), and elastomers.

[0041] Generally, the cured articles prepared in accordance with thepresent invention have a specific gravity which is lower thanstandard-density composite rocket nozzles. For instance, with a suitablecarbon phenolic resin-based pre-preg, a lower density (SpG) in a rangeof about 1.00 (e.g. 1.03) to about 1.15 g/ml may be prepared, and forother pre-pregs, such as a silica cloth pre-preg, composite rocketnozzle components having densities (SpG) of about 1.35 to 1.49 may befabricated. Further, the cured articles of the present invention alsoexhibit across-ply tensile strengths which may be greater than, or atleast near to, those observed with well-prepared composites fromstandard-density pre-pregs. For instance, the composite rocket nozzlesof the invention may have an across-ply tensile strength of about 1800to about 3000 psig and, more particularly, may be readily prepared tohave across-ply tensile strengths of 1800 to 2200 psig.

[0042] In accordance with one embodiment of the present invention, theabove-mentioned (a) may be performed by winding a suitable pre-pregmaterial about a mandrel using a tape wrap process as shown in FIG. 2.In the illustrated tape wrap embodiment, a pre-preg material or tape tobe wrapped is in rolled form about a reel or spool 10. The tape isunwound from spool 10 and passed between a roller 11 and a mandrel 12.In the illustrated embodiment, the mandrel 12 rotates clockwise and theroller 11 rotates counterclockwise as the tape to be wrapped about themandrel 11 is fed therebetween. Each of the roller 11 and the mandrel 12has a longitudinal axis; the longitudinal axes of the roller 11 andmandrel 12 are situated in parallel relationship with respect to eachother. The pre-preg tape is heated, such as by a hot air source, as itcrosses over the roller 11 in order to soften the tape. The hot air maybe supplied to the tape at a temperature in the range of about 350 toabout 900° F., but, in present practice, a temperature in the range ofabout 350 to 550° F. has proven sufficient. The pressure is applied tothe tape at the interface of the roller 11 and the mandrel 12 debulksthe tape as it wraps onto mandrel 12. The pressure applied by the roller11 and the mandrel 12 is generally less than about half (50%) of thelowest pressure recommended by pre-preg vendors (100 to 300 pounds perinch of tape width) and about 50 pounds per inch of tape width isexemplary of such a lower pressure. The tape is cooled and preferablyhardened as the tape is wrapped about the mandrel 12. As shown in FIG.2, carbon dioxide (or other coolant) from a liquid holding tank isapplied to the wrapped tape as the tape wraps about the mandrel 12. Thiscools and hardens the wrapped tape about the mandrel whereby a wrappedbillet is formed.

[0043] As mentioned above, (b) involves curing the formed pre-preg at aselected temperature and pressure. The pressure may be vacuum or up toless than about atmospheric pressure. In accordance with one embodimentof the present invention, the wrapped billet may be vacuum bagged usinga vapor impermeable vacuum bag, such as a nylon vacuum bag, sealed withvacuum putty. In the vacuum bagging procedure, a suitably thick orsuitably layered bleeder material is used so that any resin bleed fromthe curing material does not clog vacuum lines and also avoids (if notminimizes) adversely affecting the vacuum bag itself. In this regard, ableeder material may comprise, for instance, at least one layer ofperforated film; at least two layers of cotton mop; or at least twolayers of bleeder material, such as a polyester bleeder material (about10 oz or about 16 oz./sq. yard of polyester bleeder material). Thevacuum bagged material is then cured using a selected maximum vacuum. Bypreference, the vacuum provides the only pressure applied during thecuring, and low pressures on the order of subatmospheric up to about,preferably less than, atmospheric may be used, such as a vacuumequivalent of about 12.5 psia to less than about 14.7 psia, although thepressure selected will depend on the prevalent atmospheric pressure.Still lower pressures may also be used. In general, however, the lowpressure vacuum may, for instance, be in a range of from about 12.5 psiato about 14 psia and, more particularly, in a range of from about 12.5psia to about 12.7 psia.

[0044] The cure pressure is adjusted to maintain or provide the desiredcured component density. The cure pressures may range, for instance,from 12.5 psia to about 150 psig, but generally a lower upper pressureis preferred, such as up to about 50 psig. A suitably applied low curepressure may be achieved by vacuum bagging the component by adapting themethodology per standard procedure for cure, but in which a vacuum ispulled throughout the cure cycle.

[0045] During the curing step, it is possible to use thermal cycling tocure the pre-preg. In the temperature cycling, various temperatures, ortemperature and residence times, are selected to ensure the satisfactorycuring of the composite article fabricated in accordance with theprocess of the present invention. An exemplary, non-limiting temperaturecycle that may be practiced in accordance with an embodiment of thepresent invention involves a thermal cycle in which the pre-preg isheated and cooled in cycles which change on the order of about 1° F. perminute. In another embodiment, a three-stage temperature cycle may beused, although other staged temperature cycling may be used within thescope of the invention. For instance, the thermal cycling may beprogrammed to have a first hold at about 180° F. for a residence time ofabout 1.25-1.5 hours, a second hold at about 220° F. for a comparableresidence time as the first hold, and a third hold at a temperature in arange of from about 310° F. to about 320° F. for about 60 or moreminutes for each inch of component wall thickness. The residence time ata holding temperature will be a function of process variables, such asthe thickness of the material to be cured. The determination of suitablecuring cyclings, including both residence times and cure temperatures,is within the purview of those skilled in the art and may be determinedwithout undue experimentation when done in reference to the instantdisclosure.

[0046] A further post-cure step may, if desired, be conducted.

[0047] Referring now more particularly to FIG. 3, press debulking in amold may be employed as an alternative to tape wrapping for performingstep (a). In the illustrated embodiment, a ply or stack of plies 31 isplaced in a platen press for debulking. Ordinarily, the platen moldcomprises a first platen 30 and a second platen 32 preheated to atemperature of about 130° F. to about 150° F. before the ply or stack ofplies 31 are placed in the mold. During the partial debulking, apressure F is applied to urge platen 30 to close to compact a ply orstack of plies of curable pre-preg 31 on second platen 32. Stops 33 areset to restrict the travel of platen 30 and to achieve the selecteddebulked thickness. The platens 30 and 32 remain closed to the stops andthe temperature is held at about 130 to 150° F. for a sufficient amountof time to allow the ply layers to stabilize, equilibrate, at thepressure and come to a relatively even temperature. The partial setachieved with the partially debulked plies is not a cured set, e.g. bypreference the debulked plies are not thermoset. For instance, ingeneral, the residence time of the ply or plies 31 in the closed platenmold is about 20 to 30 minutes when most rocket nozzle components arebeing fabricated and, particularly, when about 30 to 40 pre-preg pliesare being used. It has been discovered that some rocket nozzlecomponents need only a relatively short time, such as about 20 minutesresidence time, in the closed platen. The partially debulked plies (ordebulked stacks of plies) are allowed to cool to room temperature andthen the platen mold is opened by separating platen 30 from platen 32.The debulked ply (plies), are obtained and then stacked to form thedesired billet. The pressure is selected to be sufficient to press ordebulk the plies to the stops.

[0048] Flat panels may be prepared using the platen mold process step todebulk the pre-preg. For instance, for a 0 degree ply angle flat panel,ply stacks may be debulked to stops (see FIG. 3) to achieve thepreselected ply thickness. Ply thicknesses may vary, but plies of 20 to22 mils may be facilely prepared. In general, debulked ply stacks areless than about 1 inch thick (less than 2.54 centimeters), such as, forexample, 0.80 inch thick perpendicular to the ply. For billets otherthan flat panel lay up, it may be desired to use different ply angles.For instance, a 45 degree ply angle may be used in a closed cylindricalmold with angled end plugs inside of the press. The debulked plies, asstated above, have a thickness of less than about 1.0 inch perpendicularto the ply. Debulking may be achieved by warming the plies and the moldto a suitable temperature, generally above 100° F., such as about 130°F. to about 150° F., while applying pressure to close the press tostops. The debulked plies are allowed to cool while the press remainsclosed to the stops. After debulking, the ply stacks are stackedtogether. Flat panels may be free standing and angle lay ups may becontained within the mold. The ply stacks are vacuum bagged and cured toa temperature cycle as described above.

[0049] The platen mold with configured (shaped) pre-composite (uncureddebulked, shaped uncured article) may, if desired, be autoclaved orplaced in an oven for curing.

[0050] Material is applied to the mandrel or debulked in a mold using apressure that yields a ply spacing which results in a composite of thedesired density. Thus, to provide desired composite density, debulkapplication pressures may be varied from just a few psig to less thanabout 800 psig. The pressures may generally be in a range of 240 psig toabout 40 psig. Importantly, in contrast to conventional debulkingtechniques, the present process uses lower pressures.

[0051] The present process is atypical and contrary to a vendorrecommended procedure for using a standard-density pre-preg. In thepresent process, tape wrap or compaction pressures are selected incombination with cure pressures that allow voids to form based on thevapor pressure of the constituents of the resin. For instance, in thetape wrap embodiment, the pressure for applying the pre-preg to themandrel is backed off sufficiently until the selected ply spacing isachieved. The debulked billet may be vacuum bagged and cured, wherebythe applied vacuum results in atmospheric pressure being applied to thebillet, providing across-ply pressure that results in a cured componenthaving an across-ply tensile strength that is greater than theacross-ply tensile strengths achieved in pre-pregs which were speciallydesigned by vendors to result in low-density composite components. Theuse of the vacuum bag technique offers the further related advantagethat volatiles evolving in the cure are capable of growing into voids inthe curing material which further reduces the density of the component.As a consequence, a conventional standard-density material may now beused to produce a low-density composite component.

[0052] Panels fabricated according to the present process using astandard-density grade carbon phenolic indicate that a cured bulkdensity as low as 1.0 to 1.15 grams per cubic centimeter may beproduced. In contrast, the same material, processed to typicalvendor-recommended process parameters which are the norm in theindustry, results in a composite component having a bulk density, of1.45 to 1.49 grams per cubic centimeter.

[0053] For instance, the present process may produce cured products fromfilled pre-pregs which have a lower density than a corresponding productprepared according to industry standard. A silica-filled phenolicpre-preg was processed to a cured article having a density of 1.53 g/mlusing the present process whereas like material processed using theindustry standard process had a density of 1.75 g/ml. A glass-filledpre-preg was likewise processed to a cured product having a lowerdensity (1.81 g/ml) verses 2.0 g/ml for a conventionally preparedarticle.

[0054] Suitable pre-preg materials are generally characterized by areinforcement (fibers, cloth, tape), which is impregnated with athermosetting resin. Suitable reinforcement may comprise a wide varietyof fibers or filaments known in the art. These fibers and filamentsinclude, but are not limited to, glass fibers, boron filaments, boronnitride, silicon carbide, graphite (carbon) filaments and high modulusorganic filaments, particularly organic filaments of the nylon,polyethylene, and aramid type. Examples of high modulus organicfilaments include, but are not limited to, poly(benzothiazoles) andpoly(aromatic amides) which are commonly referred to simply as“aramids.” Aramids include poly(benzamides) and a family of materialssold by E.I. DuPont under the trademark KEVLAR. As an example of carbonfilaments useful in this invention, there may be mentioned, for example,Amoco's Performance Product T-300 and Toray's T-800H and T-1000G carbonfibers. Suitable thermosetting resins include, for example, phenolicresins, and epoxy resins (especially those based on diglycidyl ethers ofbisphenol A are employed). In principle, bis-maleimide resins,polyurethanes, polyesters, and the like, and any combination thereof mayalso be employed as the base resin or a component thereof.

[0055] By preference, for the composite materials requiring erosionresistance, good ablative performance, and good across-ply tensilestrength required for rocket nozzle components, a phenolic resin matrixresin for the pre-preg is used. Suitable commercially available curablephenolic resins are SC-I008 (Borden) and 91-LD phenolic resin(Stuart-Ironsides).

[0056] Epoxy-based pre-pregs are not preferred for rocket motor nozzlecomponents.

[0057] A suitable pre-preg is, in general, pliable to enable it to bewrapped. Pliability is generally observed prior to debulking and curing.

[0058] The pre-preg materials may, if desired, include filler materials.Suitable filler materials include silica, carbon powder and others knownto those skilled in the art. Filler materials may serve more than onefunction. For instance, some fillers, including powdered aluminatrihydrate or antimony oxide, may also provide some flame resistance, orother characteristics to the final cured products, but are nonethelessfillers. The present process does not require, and preferably avoids theuse of hollow microspheres (and elastomers) in order to produce alow-density composite article.

[0059] Various pre-preg materials are suitable for use herein. A carboncloth impregnated with a curable phenolic resin may be used. The carboncloth may be rayon-based, polyacrylonitrile- (PAN-) based, orpitch-based. These types of pre-pregs include the industrystandard-density pre-pregs such as MX-4926 (28-38% resin, 8-16% filler,the remainder including cloth reinforcement) which is a rayon-basedcarbon phenolic pre-preg from Fiberite. Other suitable materials fromFiberite include PAN-based pre-pregs, such as those sold under thedesignations MX-4946 or MX-4920 which both used a T300 fiber/yarn whichis a vendor designation for a yarn having a modulus of 300 million.Other “T” type fiber/yarn products may be used. These materials mayinclude a filler, and generally include carbon powder filler. Graphitecloth phenolics may be used. The graphite fiber may be rayon-, PAN- orpitch-based. Suitable commercially available graphite pre-pregs includeFiberite products sold under the designations MXG-175 (graphite which israyon based) and MX-4961 (graphite which is PAN-based). Glass clothpre-pregs include a glass fiber reinforced phenolic resin such asMXB-6001 from Fiberite. Silica fiber reinforced phenolic resin, such asa product from Fiberite known as MX-2600, may be used.

[0060] The following Table 1 lists some of the pre-preg materials andprovides further characterization of the same. The fabric weave may bedifferent between suitable pre-pregs. In Table 1, “HS” stands forharness satin weave, e.g. 8HS means 8 harness satin weave, etc., whereasa simple weave pattern comprises one over, one under, one over. With PANmaterials, designations “3K,” “6K” and “12K” refer, respectively, to3000, 6000 and 12,000 filaments in a single yarn used in the pre-preg.TABLE 1 Material Characteristics Billet Material Density, DesignationReinforcement Resin System Filler gm/cc Processing Methods Status/UseMX-4926 Rayon-based SC-1008 Phenolic C 1.45 gm/cc Tape wrapping, Used innozzles on (Fiberite) Carbon Fiber, 8HS Resole (Borden) compressionmolding RSRM boosters, D-5, PK, MM, C-4, Castor 120 GT, Castor 4a, Star,etc. FM-5055 (BP) Rayon-based 91-LD Phenolic C 1.45 gm/cc Tape wrapping,Same as MX-4926 Carbon fiber, 8HS Resole (Ironsides) compression moldingMX-4946 PAN-based Carbon SC-1008 Phenolic C 1.55 gm/cc Tape wrapping,Currently used in (Fiberite) fiber (T-300), 6K, (Borden) compressionmolding Castor 120 GT 5HS MX-4920 T-300 Commercial, SC-1008 C 1.55 gm/ccTape wrapping, Low-cost material (Fiberite) 12K, 4HS compression moldingUnfilled T-300 Commercial, SC-1008 — ≈1.4 gm/cc Tape wrapping, Low-costmaterial MX-4920 12K, 4HS compression molding Fiberite MX-134 LDR T-300Plain weave, NBR modified C, MB ≈1.1 gm/cc Tape wrapping, Lower costlow-density (Fiberite) 6K SC-1008 compression molding material Fiberite

[0061] In the foregoing table, the materials with MB, microballoons, maybe used, but are not preferred because the composites produced therewithdo not exhibit the overall balance of favorable properties forcomposites produced using standard-density pre-pregs according to thepresent invention. Thus, the present invention does not require the useof the conventional specially formulated pre-pregs for low density. Asmay be mentioned elsewhere herein, these low-density pre-pregs include alow-density carbon cloth phenolic, such as one in which the resin isfilled with microballoons/microspheres, carbon filler and, optionally,an elastomer additive, such as MX-4926 LDC (rayon-based carbon fiber)from Fiberite. A low-density glass fiber-reinforced phenolic resinloaded (filled) with microballoons/microspheres is known as MXS-385LDfrom Fiberite. A low-density silica cloth phenolic filled withmicroballoons/microspheres is known, and may also have anelastomer-modified resin, such as the specially formulated low-densitypre-preg known as MX-2600LD from Fiberite.

[0062] Final cured and shaped products producible by the present processinclude rocket nozzle components. The present process may also be usedto manufacture composite panels. For instance, a standard-densitycarbon-fiber cloth impregnated with a curable phenolic resin may be usedto prepare a panel having, when cured, a bulk density of 1.0 g/ml to1.15 g/ml. This type of panel is more cost-effective than a panel or,for instance, other part, such as an aft exit cone, for a C4 missile.

[0063] The present process may be used to produce a specific tailoreddensity composite article of manufacture, such as rocket nozzlecomponents. Rocket motor components include, among others, blast tubes(aft, mid and/or forward); nozzle throats, exit cones and suchcomponents as depicted in FIG. 17. For instance, across-ply propertiesof the composite material may be tailored by adjusting the pressure usedin the initial compaction step and pressure used in the cure step. Suchadjustments are techniques that, when viewed in reference to thisdisclosure, would be within the purview of the skilled artisan withoutundue experimentation.

[0064] Composite components were fabricated according to the tape wrapand platen press embodiments of the present invention. Products madeusing the platen press embodiment were evaluated and low-densityproducts with bulk densities in the range of 1.05 to 1.13 g/ml werefabricated. The products were tested in an FPC motor using a propellantfor the Space Shuttle Solid Rocket Motor. Firing time in the tests ranfrom about 30 to 35 seconds, with average pressures in the range ofabout 650 psig to about 750 psig. These products were compared to aconventionally produced standard-density product. The results of thiscomparison are reported in Table 2. TABLE 2 Property Invention MX-4926MX-4926LD Bulk Density (g/ml) 1.05-1.13 1.45-1.48 1.0-1.15 Across-PlyTensile Strength 2200 3000 700 (psi) Erosion rate (mils/sec) 7.6 7.313.5

[0065] The data show that orders of magnitude improvement in tensilestrength and significant improvement in erosion resistance areattainable with composite rocket nozzle components prepared according tothe present process. One of the unexpected beneficial attributes is theover 400% improvement in across-ply tensile strength with the presentlow-density composite products compared to the conventional low-densitycomposite products.

[0066] Testing the low-density rocket nozzle components using the fortypound charge rocket nozzle demonstrated that the vacuum cured materialhad the desired combination of erosion resistance, charring, andacross-ply tensile strength associated with a standard-density compositecomponent prepared using the conventional procedure. However, thecomposites offered the additional advantage of lower weight compared toconventional composites that may have comparable erosion resistance.TABLE 3 Performance Comparisons, FPC Nozzle Components Cured Com-Erosion Heat Bulk pressive Rate Char Affected Density Strength (mils/Depth, Depth Material Description (g/cc) (psi) second) (inch) (inch)Vacuum-cured 1.14 19750 7.59 .258 .534 MX4926, 45° Blast Tube(rayon-based) Traditional MX4926, 1.45 41850 7.59 .246 .502 45° BlastTube (rayon-based) Vacuum-cured 1.38 23067 6.18 .464 .680 MX4920, 45°Blast tube (PAN-based) Traditional MX4926, 1.53 — 4.60 .461 .656 45°Blast tube (PAN-based) Low-Pressure 1.10 — 5.42 .269 .459 MX4926, 30°Exit Cone (rayon-based) Traditional MX4926, 1.46 — 4.22 .193 .341 30°Exit Cone (rayon-based) Low-Pressure 1.42 — 3.05 .219 .326 MX4920, 30°Exit Cone (rayon-based) Traditional 1.34 — 4.5  .372 .530 MX134LDR, 30°Exit Cone (PAN-based)

[0067] It will thus be seen that the objectives and principles of thisinvention have been fully and effectively accomplished. It will berealized, however, that the foregoing preferred specific embodimentshave been shown and described for the purpose of this invention and aresubject to change without departure from such principles.

What is claimed is:
 1. A composite article comprising: a pre-pregmaterial comprising a reinforcement impregnated with a thermosettingresin, the composite article having a specific density ranging fromapproximately 1.00 g/ml to approximately 1.15 g/ml.
 2. The compositearticle of claim 1, wherein the thermosetting resin comprises a carbonphenolic resin.
 3. The composite article of claim 1, wherein thethermosetting resin comprises a phenolic resin or an epoxy resin.
 4. Thecomposite article of claim 1, wherein the reinforcement comprises glassfibers, boron filaments, boron nitride, silicon carbide, graphite(carbon) filaments, or high modulus organic filaments.
 5. The compositearticle of claim 4, wherein the high modulus organic filaments comprisepoly(benzothiazoles) or poly(aromatic amides).
 6. The composite articleof claim 1, wherein the reinforcement comprises organic filaments ofnylon, polyethylene, or aramid.
 7. The composite article of claim 1,wherein the pre-preg material further comprises a filler material. 8.The composite article of claim 7, wherein the filler material comprisessilica, carbon powder, powdered alumina trihydrate, or antimony oxide.9. The composite article of claim 1, wherein the composite articlecomprises a rocket nozzle component.
 10. The composite article of claim1, wherein the composite article comprises a composite panel.
 11. Thecomposite article of claim 1, wherein the composite article has anacross-ply tensile strength of about 1800 psig to about 3000 psig. 12.The composite article of claim 1, wherein the composite article has anacross-ply tensile strength of about 1800 psig to about 2200 psig.